Color imaging systems known in the art permit images to be captured by certain color-imaging media, possibly digitized and stored, and then output onto complementary media. So, for instance, color images may be first captured on negative-working silver-halide-based photographic film and then reproduced on negative-working photographic paper. Such images may or may not pass through a digital intermediary. In another case, color images may be captured on positive-working photographic materials, known as transparencies, and then viewed directly by projection or back-illumination, or copied onto larger or smaller transparencies, or printed onto positive photographic paper. Again, such images may or may not pass through a digital intermediary.
Color-imaging systems in which the image passes through a digital intermediary are often referred to as "hybrid" imaging systems because they combine elements of photographic or other chemical-based imaging together with various elements of electronic imaging systems. Such systems may offer advantages such as convenient image modification, image editing, and image storage.
A hybrid imaging systems must include a method for scanning or for otherwise measuring the individual picture elements of the photographic media, which serve as input to the system, to produce input image-bearing signals. In addition, the system must provide a means for transforming the input image-bearing signals produced by the input scanning device to intermediary image-bearing signals, i.e., to an image representation or encoding that is appropriate for the subsequent applications of the system. Accurate transformation of the input image-bearing signals to and from the encoded image representation requires calibration of the various input and output devices of the system. Techniques for such calibration are well know to those skilled in the art.
For example, U.S. Pat. No. 4,500,919 entitled "COLOR REPRODUCTION SYSTEM" by W. F. Schreiber, discloses an image reproduction system of one type in which an electronic reader scans an original color image and converts it to electronic image-bearing signals. A computer workstation and an interactive operator interface, including a video monitor, permit an operator to edit or alter the image-bearing signals by means of displaying the image on the monitor. When the operator has composed a desired image on the monitor, the workstation causes the output device to produce an inked output corresponding to the displayed image. In that invention, calibration procedures are described for transforming the image-bearing signals to an image representation or encoding so as to reproduce the colorimetry of a scanned image on the monitor and to subsequently reproduce the colorimetry of the monitor image on the inked output.
U.S. patent application Ser. No. 059,060 entitled "METHODS AND ASSOCIATED APPARATUS WHICH ACHIEVE IMAGING DEVICE/MEDIA COMPATIBILITY AND COLOR APPEARANCE MATCHING", by E. Giorgianni and T. Madden describes an imaging system in which image-bearing signals are converted to a different form of image representation or encoding, representing the corresponding colorimetric values that would be required to match, in the viewing conditions of a uniquely defined reference viewing environment, the appearance of the rendered input image as that image would appear if viewed in a specified input viewing environment. The described system allows for input from disparate types of imaging media, such as photographic negatives as well as transmission and reflection positives. In that invention, images are digitally encoded in terms of the color appearance of the image being scanned (or of the rendered color appearance computed from a negative being scanned), and calibration procedures are described so as to reproduce that color appearance on the monitor and on the final output device/medium.
The colorimetric image representation or encoding described by Schreiber is appropriate and desirable for applications where the intent is to represent the colorimetry of an image reproduced directly on, or to be subsequently produced from, a color-imaging medium being scanned into the imaging system. The color-appearance image representation or encoding described by Giorgianni/Madden is appropriate and desirable for applications where the intent is to represent the color appearance of colors as reproduced directly on, or to be subsequently produced from, various color-imaging media scanned for input to the system. In each of these descriptions, the photographic image being scanned is taken to be the original to be reproduced. While calibration is described in each of these systems to allow the appropriate reproduction of the scanned image, neither system provides or requires calibration of the input photographic medium itself, i.e., calibration which would describe the relationship of the scanned image to the original scene or other source of exposure which caused the photographic image to form. Because each system treats the scanned photographic image as the original to match, such input-medium calibration is not required.
It is well known to those skilled in the art, however, that the colors reproduced on, or produced from, a photographic color-imaging medium generally are not colorimetric matches of the actual colors originally photographed by the medium. Colorimetric differences can be caused by the color recording properties of the medium, i.e., its formation of a latent image in response to exposure. Colorimetric errors can also be produced by the color reproduction properties of the medium, i.e., properties related to color image formation subsequent to color image recording. These reproduction properties include the characteristics of the medium's chemical signal processing, such as the relationship between exposure and dye formation within each layer and the chemical relationships among the various layer of the medium. Color reproduction is also influenced by the colorimetric properties of the image-forming dyes of the medium.
In certain hybrid imaging applications, it is not desirable to represent the colors of the image as they appear on, or as they are produced from, the color-imaging medium being scanned into the system. In such applications, it would instead be desirable to form image representations that correspond more closely to the colorimetric values of the colors of the actual original scene that was photographed by the color-imaging medium rather than to image representations that correspond to the reproductions of those colors by the medium itself. Examples of such applications include, but are not limited to, the production of medical and other technical images, product catalogues, magazine advertisements, art-work reproductions, and other applications where it is desirable to obtain color information which is a colorimetrically accurate record of the colors of the original scene. In these applications, the alterations in the color reproduction of the original scene colors by the color recording and color reproduction properties of the imaging medium are undesirable, and the previously described image representations of the prior art are, therefore, also undesirable.
A hybrid imaging system can provide the capability to produce image representations or encodings that represent original scene colorimetric information. A system employing this type of image representation or encoding could be used to form and store a colorimetrically accurate record of the original scene and/or used to produce colorimetrically accurate or otherwise appropriately rendered color images on output devices/media.
In order for an imaging system to accurately represents original scene colorimetric information, its image representation or encoding must not include color alterations produced by the color reproduction properties of the imaging medium. U.S. Pat. No. 5,267,030 entitled, "METHODS AND ASSOCIATED APPARATUS FOR FORMING IMAGE DATA METRICS WHICH ACHIEVE MEDIA COMPATIBILITY FOR SUBSEQUENT IMAGING APPLICATIONS", by E. Giorgianni and T. Madden, provides a method for deriving, from a scanned image, recorded color information which is free of color alterations produced by the color reproduction properties of the imaging medium. In that patent, a system is described in which the effects of media-specific signal processing are computationally removed, as far as possible, from each input medium used by the system. In addition, the chromatic interdependencies introduced by the secondary absorptions of the image-forming dyes, as measured by the responsivities of the scanning device, are also computationally removed. Consistent with the input media compatibility objectives of that invention, each input image is transformed to an image representation or encoding corresponding to the exposures recorded from the original scene, or other source of exposure, which caused the image to form on the input imaging-recording medium.
In that invention, the extraction of recorded exposure information from each input medium allows for input from disparate types of imaging media, such as conventional photographic negatives and transmission and reflection positives. That same process of extracting recorded exposure information can also be use to effectively eliminate any contribution to color inaccuracy caused by chemical signal processing and by the image-forming dyes.
However, the elimination of color inaccuracies caused by chemical signal processing requires a foreknowledge of such processing, i.e., a knowledge of the relationship between the latent image exposure recorded by the medium and the amounts of the image-forming dyes, or the resulting optical densities, that are produced from that exposure. Those skilled in the art will recognize that the density vs. exposure relationship is subject to variability that may be caused by variations in the manufacturing of the photographic medium, by changes that occur after the manufacturing of the photographic medium, by variations in the photographic development and other chemical processing of the medium, and possibly by other factors.
It is well known to those skilled in the art that the density vs. exposure relationship for a chemically processed photographic medium can be determined using appropriate calibration procedures. The basic procedure begins with the exposure of a test sample of the photographic medium to a pattern of known exposures. After the film is exposed to a pattern, the medium is chemically processed, and the resulting optical densities are measured and related to their corresponding exposures. There are two types of patterns generally used; a) a continuous wedge that generates exposures from a relatively high exposure value to no exposure in a continuously varying fashion, and b) an array of spatial patches with stepped exposures of known increments of increasing or decreasing exposure going from one end of the array to the other. The stepped alternative (b) is preferred in the trade and is used in this document since this type of pattern has positional indicators at each step rather than only one at the end of the continuously varying pattern as in alternative (a).
In some applications, it would be desirable to include a calibration exposure pattern on the actual photographic medium that is to be used for recording images, rather than on a separate test sample of that medium. It may also be desirable to locate the pattern of exposure as close as possible to the image area. For example, in U.S. Pat. No. 3,718,074, Davis describes a camera that includes apparatus for exposing patterns of exposures on a photographic medium at the time of scene exposure. The patterns of exposures may be produced using the ambient light source or a calibrated light source included in the camera.
In the Davis patent, exposure patterns comprised of five spatially separated areas of exposure are used. However, it is well known that a precise determination of the density vs. exposure relationship requires that the pattern of exposure be comprised of a greater number of exposure levels. For example, a series of twenty or more exposure levels is typically used for calibrating a photographic medium having a highly nonlinear density vs. exposure relationship.
The use of exposure patterns comprised of a relatively large number of exposure levels may require the use of a considerable area of the photographic medium. The total exposed area can, of course, be minimized if each area of exposure is small. However, it is well known to those skilled in the art that edge density artifacts, such as chemical adjacency effects, can alter the density vs. exposure relationship of small areas of exposure. For example, Dainty and Shaw in Image Science, Academic Press, 1974, pp 53-55, state that densities change not only as a function of exposure but also as a function of exposure area in areas smaller than 1.0 mm square. As a result, a density vs. exposure calibration determined from patterns of exposures where each individual area in the pattern is small may not be representative of the actual density vs. exposure relationship for the medium.
Relatively large areas of exposure are therefore traditionally used for the calibration of photographic media in order to avoid the misleading results that can be produced by chemical adjacency effects. In conventional test exposures, each exposure area is typically 1 cm in width and at least 1 cm in length.
An exposure pattern consisting of a relatively large number of exposure areas in which each exposure area is greater than 1.0 mm square may be too large for many practical applications. For example, the total area would be too large to include within the normally unused area of a frame of standard 35 mm film.